A pg_default_acl entry shows the
initial privileges to be assigned to an object belonging to the
indicated user. There are currently two types of entry:
"global" entries with defaclnamespace = 0, and "per-schema" entries that reference a particular
schema. If a global entry is present then it overrides the normal hard-wired
default privileges for the object type. A per-schema entry, if
present, represents privileges to be added to the global or hard-wired
default privileges.

Note that when an ACL entry in another catalog is null, it is
taken to represent the hard-wired default privileges for its
object, not whatever
might be in pg_default_acl at the
moment. pg_default_acl is only
consulted during object creation.